As it is known from the prior art, diesel engines subjected to emission standards are or are going to be equipped with particulate filters so as to meet these standards. The soots accumulate on the walls of the particulate filter. In order to provide proper gas circulation in the exhaust system and therefore smooth operation of the engine, these soots have to be regularly burned during an <<active regeneration>> period. This active regeneration is achieved by raising the temperature of the particulate filter. In most configurations, this temperature rise is carried out by means of oxidation of hydrocarbons injected into the exhaust system: an oxidation catalyst is placed upstream from a particulate filter; the hydrocarbons can notably be supplied by the main injection system of the engine or by a specific injector arranged between the exhaust manifold and the oxidation catalyst inlet.
In the diesel engine post-treatment system comprising a particulate filter, it is very important to properly control the temperature of the gas flowing into the particulate filter at the risk of damaging this filter. It is therefore very important to control the temperature of the gas leaving the oxidation catalyst.
The technical problem is as follows. The goal is to maintain, during active regeneration periods, the temperature of the oxidation catalyst close to a set point temperature under all the conditions of use of the vehicle. When using the vehicle, the external conditions imposed on the oxidation catalyst are rapidly and highly variable. It is therefore difficult to solve the aforementioned problem.
There are methods that are currently used to solve this problem.
The most commonly used method consists in calibrating a conventional controller (PI, PID, Smith predictor, internal model, etc.), This calibration is carried out for a large combination of stationary operating conditions. When using the control, the operating conditions are detected and the corresponding parameters of the regulator are applied.
A precompensation term is commonly added to this method. This term is evaluated notably as a function of the difference between the inlet temperature and the set point temperature, as described in document U.S. Pat. No. 7,047,729 B2.
However, current methods involve some performance limitations because: they do not take finely account of the phenomenologic differences between the effect of hydrocarbons on the oxidation catalyst and the effect of the temperature upstream from the oxidation catalyst; they take into account, in a limited way, the dynamic aspect of the effects linked with the gas flow rate variations; they take into account, in a limited way, the gas flow rate variations for dynamic adaptation of the controller parameters. Furthermore, current methods exhibit limitations in terms of calibration effort because they do not express the controller parameters from quantities resulting from physical modelling of the oxidation catalyst.
The object of the invention is an alternative method for treating the exhaust gas of an internal-combustion engine, wherein active regeneration of a particulate filter is controlled by means of dynamic control of the temperature at the outlet of an oxidation catalyst. The method overcomes the limitations of the prior art by means of a control law comprising:                a term taking into account transient phenomena that occur in the oxidation catalyst during a gas flow rate variation,        or a precompensation term taking into account the dynamic differences between the effect of the temperature disturbance at the oxidation catalyst inlet and the effect of the control of hydrocarbon injection upstream from the oxidation catalyst,        or a feedback term based on physical modelling of the oxidation catalyst allowing the controller parameters to be expressed as a function of the parameters of the physical model and taking dynamically into account the adaptation of the controller parameters during operating conditions variations,        or any combination of these three terms.        
The invention also relates to an internal-combustion engine comprising a control system suited for implementing the methods according to the invention.